1. Field of the Invention
The present invention relates to a method and an apparatus for bonding with sheet-like adhesives, more particularly, to a method and an apparatus that enables the sheet-like adhesives to be bonded to various types of adherends without leaving voids at the interface.
2. Description of the Prior Art
Adhesives having versatile functions and shapes are currently used in a wide range of industrial fields to achieve various purposes. These many adhesives are in most cases spread in thin layers on supports to take a sheet-like form. The resulting sheet-like adhesives are bonded to various adherends depending on the specific purpose of use.
Not only in the use of sheet-like adhesives but also in the manufacture of adhesive products, the bonding of sheet-like adhesives is a common practice in both consumer and industrial areas. For example, polyvinyl adhesive tapes which are the most common sheet-like adhesive are shipped in a roll form. Such adhesive tapes are usually manufactured by applying adhesive solutions onto sheets of soft polyvinyl chloride resins and drying the applied coasts and the resulting sheets (sheet-like adhesives) are rolled on themselves, with the adhesives being bonded to the back side of polyvinyl chloride resin sheets.
Forming adhesives on supports to make sheet-like adhesives which are then bonded to release bases, or first forming adhesives on release bases to make sheet-like adhesives which are then bonded to supports, is a common practice in the ordinary manufacturing process.
To give a further example, double-coated adhesive tapes are manufactured by bonding two sheet-like adhesives together, with paper being interposed as a support. Thus, it is a common and routine practice in the use and manufacture of adhesive products to bond sheet-like adhesives to various kinds of adherends.
FIG. 5 is a schematic representation of adhesive tape manufacture in an exemplary case of bonding a film-like support (support film) to a sheet-like adhesive having an adhesive formed on a release base. In a bonding apparatus generally indicated by D in FIG. 5, a roll of support film 2 is unwound and fed into the nip between a pair of bonding rollers 4 past a roller 5 under a specified tension. A sheet-like adhesive 1 having an adhesive formed on a release base is similarly fed into the nip between the same pair of bonding rollers 4 past a roller 3 under a specified tension. The thus fed support film 2 and sheet-like adhesive 1 are held between the bonding rollers 4 so that they are bonded together by a specified bonding force. The resulting adhesive tape 7 is past under two rollers 6 to be wound up in a roll form.
Should voids occur between the sheet-like adhesive and the support providing the adhered during the manufacturing process just described above, various inconveniences will result. For example, extensive formation of voids will induce lower adhesion strength, which eventually affects the anchoring force as exemplified by separation of the sheet-like adhesive from the support during use of the finished product.
The following additional examples may be mentioned: if voids occur when the adhesive tape is attached to a smooth surface, the back side of the tape will have asperities which give only an undesirable appearance to the tape; if voids occur between the tape and the surface of an electronic component to be fixed, the dimensional precision of the finished product will be lost; with a rough adherend, voids will turn into an infinite number of air bubbles, which again cause poor appearance.
The occurrence of voids at the interface between a sheet-like adhesive and the adherend is by no means exceptional but frequently observed in ordinary bonding procedures. Consider, for example, the case of bonding a cellophane adhesive tape onto a smooth glass plate; this would appear to be a comparatively simple job, but in reality it involves considerable difficulty in bonding the tape manually to ensure that no voids (air bubbles) will be entrapped between the glass surface and the adhesive side. If the bonding operation is performed mechanically, voids are less likely to occur than when it is performed manually but, depending on the operating conditions or the type of the adhesive used and the adherend to which it is applied, voids may occasionally occur as when excessive vibrations occur to the bonding rolls, used for continuous bonding, or when the bonding pressure is weak and not uniform or when the adherend is a rough surface.
If the occurrence of such voids is not tolerable, appropriate actions are taken depending on the specific cause of the trouble. If the cause is the vibration of bonding rollers and other components of the bonding machine, it is effective to increase the precision and strength of the machine; if the bonding pressure is unduly weak, setting it on the higher side is effective; if the adherend is a rough surface, it would be effective to set the bonding pressure at a higher level, set the bonding speed at a slower value or taking any other action to increase the chance of the adhesive of getting into the low spots of the irregularities of the adherend.
However, these conventional methods not only are limited in effectiveness but also economically disadvantageous as typified by high initial investment cost.
Particularly in the case of a rough adherend, it is difficult to eliminate all voids present by the aforementioned mechanical actions. In addition, if the rough adherend has a complex shape as in the case where it is formed of a fibrous material, it is extremely difficult to ensure that the voids which occur in the bonding operation are totally eliminated by the above-described mechanical treatments.